Processes and apparatus for embossing precision optical patterns such as microcubes, in a resinous sheet or laminate, are well known as referenced in U.S. Pat. Nos. 4,486,363; 4,478,769; 4,601,861; 5,213,872; and 6,015,214, which patents are all incorporated herein by reference. In the production of such synthetic resin optical sheeting, highly precise embossing is required because the geometric accuracy of the optical elements determines its optical performance. The above referenced patents disclose particular methods and apparatus for continuously embossing a repeating retro-reflective pattern of fine or precise detail on one surface of a transparent thermoplastic material film to form the surface of the film into the desired microstructure pattern.
Besides precision optical sheeting, various other applications have been developed requiring the formation of highly precise shapes and structures in resinous film. Such applications include (in addition to optical applications) micro-fluidic, micro-electrical, micro-acoustic, and micro-mechanical applications. Such applications require the embossing of thermoplastic material to provide precisely formed functional geometric elements, or arrays of such functional geometric elements on the film surface.
These geometric elements, or precision microstructures, are defined by any or all of the following characteristics: precise embossing depths; flat surfaces with precise angular orientation; fine surface smoothness; sharp angular features with a very small radius of curvature; and precise dimensions of the elements and/or precise separation of the elements, within the plane of the film. The precise nature of the formed surface is critical to the functional attributes of the formed products, whether used for microcubes or other optical features; or as channels for microfluidics, or in fuel cells; or for accurate dimensions, flatness and spacing when providing a surface for holding nanoblocks in fluidic self assembly (FSA) techniques; or imparting a microtextured surface that is not optically smooth.
U.S. patents describing some uses of precise microstructures include: U.S. Pat. Nos. 4,486,363; 6,015,214 (microcubes); U.S. Pat. Nos. 5,783,856; 6,238,538 (microfluidics); and U.S. Pat. No. 6,274,508 (FSA).
As described in some of the above mentioned patents, such as U.S. Pat. Nos. 4,486,363, 4,601,861, and 4,478,769, embossed microstructure film may be made on a machine that includes two supply reels, one containing an unprocessed film of thermoplastic material, such as acrylic or polycarbonate, or even vinyl, and the other containing a transparent and optically smooth plastic carrier film such as Mylar, which should not melt or degrade during the embossing process. These films are fed to and pressed against a heated embossing tool that may take the form of a thin endless flexible metal belt. The belt creates the desired embossed pattern on one surface of the thermoplastic film, and the carrier film makes the other surface of the thermoplastic film optically smooth.
The belt moves around two rollers that advance the belt at a predetermined linear controlled speed or rate. One of the rollers is heated and the other roller is cooled. An additional cooling station, e.g. one that blows cool air, may be provided between the two rollers. Pressure rollers are arranged about a portion of the circumference of the heated roller. Embossing occurs on the web as it and the tool pass around the heated roller and while pressure is applied by one or more pressure rollers causing the film to be melted and pressed onto the tool. A backing film such as Mylar® may be used in order to create an optically smooth surface on the non-embossed surface of the film. The embossed film, (which may have been laminated to other films during the embossing process), is cooled, monitored for quality and then moved to a storage winder. At some point in the process, the Mylar® film may be stripped away from the embossed film.
The prior apparatus and process work well to produce rolls of film that are effectively 48″ (122 cm) wide (52″/132 cm at salvage), but such equipment and processes have several inherent disadvantages. The process speed (and thus the volume of material) is limited by the time needed to heat, mold and freeze the film. Also, the pressure surface area and thus the time available to provide adequate pressure by the pressure rollers, and then cooling the material, impose certain special constraints.
The prior apparatus and process of U.S. Pat. Nos. 4,486,363, 4,601,861, and 4,478,769, and other embossing processes discussed below, depend on heating a preformed synthetic resinous sheeting above its glass transition temperature or melting temperature in order to emboss the sheeting while in a molten state. The embossing apparatus includes a heated roller with internal passages for circulation of hot oil. Typical temperatures of the heated roller are 425° C. to 475° C., possibly as high as 500° C. U.S. Pat. No. 4,486,363 also includes a limited disclosure (without explanatory details) of an alternative embodiment using an infrared heater or other radiant heater.
One earlier prior device for forming microcubes while in a planar condition is illustrated in U.S. Pat. No. 4,332,847, and involves indexing of small (9″×9″ or 22.86 cm×22.86 cm) individual molds at a relatively slow speed (See Col. 11, lines 31-68). That process is not commercially practical because of its perceived inability to accurately reproduce microstructures because of indexing mold movement and the relatively small volume (caused by mold size) and speed. Also, the equipment and process is non-continuous. The '847 patent discloses the use of platens that are heated by electric cartridge heaters, as well as platens that are heated using hot oil.
U.S. Pat. No. 5,945,042 discloses an embossing apparatus in which synthetic resin sheeting is fed to a thermoforming zone while in a “flow temperature region” of the resinous material; this can be accomplished by extruding molten resinous sheeting to feed to the thermoforming zone, or by pre-heating preformed resinous sheeting. The '042 Patent discloses, as to means for pre-heating the resinous sheeting, passing the sheeting between two heated rollers. It is said that “indirect heating devices such as a hot blast heater, a near-infrared lamp heater and a far-infrared lamp heaters may be used in combination”. The '042 patent also discloses heating the thermoforming roll from within using dielectric heaters or using a heated circulating medium. This heat source during thermoforming can be supplemented by “auxiliary means . . . such as a hot blast heater, a near-infrared lamp heater and a far-infrared lamp heater”. No specifics are given as to these heat sources and their operation in heating the sheeting.
U.S. Pat. No. 6,096,247 discloses a process and apparatus for making an embossed optical polymer film. A heat flux is provided by either a flame burner or a flameless radiant burner to soften at least one surface of a polymer film. The film then is passed through an embossing nip to form embossments on the softened surface of the film. This embossed surface is then cooled to fix the structure of the embossments. It is said that the time required to heat, emboss, and cool the embossed optical polymer film ranges from about 0.05 to about 1 second, depending in part on the temperature sensitivity of the optical film being embossed.
International patent application PCT/US01/18655 (publication no. WO 01/98066) discloses a process and apparatus for forming thermoplastic products having precise embossed surfaces, using a continuous double band press. This apparatus includes continuous flat beds with two endless bands or belts, preferably of steel, running above and below the product and around pairs of upper and lower drums or rollers. An advantage of such presses is the mainly uniform pressure that can be provided over a large area. This machine forms a pressure or reaction zone between the two belts and has the advantage that pressure is applied to a product when it is flat rather than when it is curved.
The machine is based upon a fluid or air cushion press, which uses a cushion of air to reduce friction between the belt and the rest of the machine. It was conceived by applicants of the above-identified application that this type of press may be suitable for precision embossing of microstructures. The fluid cushion press is sometimes associated with the term “isobaric.” In an isobaric press, heat generally does not come from rollers or drums; rather the fluid, i.e. air, provides it. The fluid transfers heat to the steel belts that in turn transfer the heat to the material passing through the press. This provides one advantage over certain prior art forms of embossing equipment—the ability to heat the film to be embossed from both sides.
It will be appreciated from the above discussion that many approaches have been undertaken with regard to embossing of microstructures, and that there is further room for improved microstructure methods and products.